1. Field of the Invention
The present invention is in the field of method and apparatus for burning cement clinker from different mineral base materials which after separate heat treatment depending on the nature of the material, are combined for formation of clinker in a clinker reaction zone.
2. Description of the Prior Art
The dry burning method is predominantly employed in the prior art for manufacturing cement, essentially portland cement, for economic reasons, particularly for reasons of heat economy. The preheating of the material is accomplished in a heat exchanger outside of a rotary tubular kiln, and the deacidification which has the relatively greatest amount of heat consumption is also carried out in a separate calciner in most instances. In this method, there is the advantage that the heat transmission to fine grains of particles in suspension with hot gas is very efficient both in the preheating as well as in the calcination stages as a result of which the specific heat consumption in cement burning is considerably reduced as compared with older installations. The sinter roasting in the rotary tubular kiln, however, still requires about 30 to 40% of the overall fuel usage when nearly complete deacidification of the raw meal is achieved in a calciner.
In accordance with the present description, the term "sinter roasting" means the heating of the product from approximately 900.degree. C. following the calcination up to a temperature in the range of incipient alite formation at approximately 1250.degree. C. as well as the range, characterized by the formation of melt phases, of sintering reaction of the lime and silicon components into technical tri-calcium silicate (alite) above 1300.degree. C. The clinker reaction or the "maturing" is considered completed when all but slight residues of free CaO have been consumed by the reaction with Ca.sub.2 SiO.sub.4 (belite).
A method for burning cement clinker which essentially corresponds to the above noted prior art can make use of a rotary tubular kiln which is preceded by two heat exchange lines operated in parallel, with raw meal being delivered to a heat exchanger which is operated with cooler exhaust gas and clay being delivered to a heat exchanger operated with the kiln exhaust gases. This type of installation is described in DE-LP No. 1,213,337. The division of heat exchange lines prevents carbon dioxide present in relatively high concentration in the kiln exhaust from reacting with free calcium oxide in the raw meal to form calcium carbonate which would result in the fact that additional heat would have to be exerted for the additional deacidification.
A similar improved installation includes a rotary tubular kiln and two preceding, parallel heat exchanger lines wherein the clay component is likewise delivered to the line charged with kiln exhaust and the lime component is delivered to the line charged with hot cooler gas. The temperature is boosted with auxiliary burners in accordance with the heat consumption. Preheated clay together with heated limestone proceed over a conduit for hot cooler air equipped with auxiliary burners and into a heat exchanger preceding the rotary tubular kiln. Both base materials are homogeneously mixed therein and are heated further at the same time.
This known process has the advantage that with an elevated sulfur content in the kiln exhaust gases, the resulting sulfur dioxide is eliminated from the system in the heat exchanger line charged with the argillaceous minerals and can be separated from the gases in a known manner. This type of system is described in DE-OS No. 22 62 213.
The tendency of further developments and improvements has been to further shorten the rotary tubular kiln and, to this end, to transfer the heating of the product in the temperature range between about 900.degree. C. and 1250.degree. C. into a preceding, stationary heating unit. A further goal of these developments is to further reduce the fuel inventory per weight unit of clinker, the energy consumption of the system including the energy consumption of the clinker grinding system, and the capital costs of the system.
Continuing improvement of the so-called "rapid burning" process could make an important contribution to the attainment of this goal if one could succeed in overcoming the difficulties, technical limitation, and problems connected therewith.
It was soon realized that the heating of the product as rapidly as possible could be achieved on the basis of intimate contact between the flame and the product, particularly in the heating phase between deacidification and alite formation in order to improve burning results. This is discussed in the early DE-LP No. 337,312 of May, 1921. A divided rotary tubular kiln was proposed as a possible solution, with a sintering part which rotated more slowly than the kiln for heating product close to the sintering point. The kiln was supposed to rotate at a sufficiently high speed so that the product was lifted close to the apex of the kiln and freely fell through the kiln cross section. Heat transmissions in the convection zone are theoretically significantly higher as a result, lying partially on the order of heat transmissions of the product in suspension. This proposal, however, was never developed because of the substantial amounts of dust arising in the system gas.
A further proposal for implementation of the so-called rapid burning of raw material was disclosed in DL-LP No. 97 409. According to this reference the powdery or agglomerated mixture could be rapidly heated in a fluidized bed in a stationary reactor and could be sintered up to maturing, whereby extremely high heating gradients are achieved in the temperature range between about 1100.degree. and 1350.degree. C. An advantage of this procedure would be a reduction of the maturing time by about 70% with advantages that flow from this, namely, reducing the size of the maturing reactor or increasing its throughput or carrying out the maturing at lower temperatures with a lower fuel inventory. Some of these advantages resulting from extremely high heating gradients are derived from avoiding deactivation of the calcined product.
Although the theoretical bases of this procedure are correct, the proposal was likewise not developed commercially because of difficulties in the stationary heating units. For example, cakings and encrustations can arise when heating the product in a stationary unit even in the temperature interval of 850.degree. to 1250.degree. C. Such encrustations become adhesive due to imbalance at the high temperatures, but are principally due to the secondary components such as KCl, K.sub.2 SO.sub.4, CaS0.sub.4, and the like, which are enriched in the circulation. Such encrustations require an expensive procedure for their removal and deteriorate the process in the final analysis.
If the foregoing problems could be overcome, and limits placed on technical feasibilities thereby caused could be expanded, considerable progress would result. This advantage results from savings of fuel per ton of clinker, savings in energy consumption, reduction of investment volume and, from a reduced requirement for grinding energy, since clinker produced by means of rapid burning is not as highly sintered but turns out to be relatively porous.
With the initially described, presently used standard burning process wherein there is countercurrent flow of burning product and system product, however, a porous clinker is extremely injurious because of the increased dust production inasmuch as dust circulations produce considerable reduction of the system performance and result in an increase of the specific energy consumption.